The winner of last Saturday's $20.5 million Lotto Powerball prize put off checking his ticket so the magic of thinking he might be a millionaire could last a little longer.

The South Island man, who wants to remain anonymous, knew he had bought his ticket from the Invercargill store that had sold the winner, but did not think he could be the winner.

"I deliberately delayed checking my ticket as I wanted the magic of thinking I could be a millionaire to last a bit longer. I didn't really think that I could be the winner, so I was completely stunned when I did look online and saw that I had right numbers," he said.

The man, who has now claimed the prize, kept the winning ticket in his wallet hidden in his sock drawer.

"I haven't really been able to sleep - I keep getting up in the night to check that it is still there. It still feels a bit unreal really."

He had contacted his bank and been blown away by how much interest he will earn.

"It's enough for me and my partner to think about stopping work. Plus I can now really indulge my liking for vintage motorcycles. We also would like to plan some overseas travel - in style," he said.

When he first rang his partner with the good news she had not believed him.

"I do play the odd joke on her, so she originally thought I was having a laugh again."

He bought the ticket on a whim while in Invercargill for the Burt Munro Challenge motorcycle event.

"It was a spur of the moment decision to get a Lotto ticket," he said. "I don't play every week but I had a spare $20 in my wallet so though I would give it a go."

The winning ticket was sold at Windsor Stationery & Lotto in Invercargill. Previous winners sold at the store include two Strike Four prizes, six Lotto first division prizes and one instant millionaire prize, as well as two Winning Wheel spinners.